Methanogenesis
While aerobic and anaerobic microbial activities of RAS
biofilters significantly decrease nitrogen and organic matter,
complete removal of dissolved and solid organic wastes
is a major challenge of an RAS operation. Solid organic
waste— suspended matter originating from uneaten feed
and fish feces — is often thickened and used for land
applications or composting. For freshwater systems, this
approach has environmental consequences and faces regulatory
scrutiny. Marine systems pose an additional complication
because of the presence of concentrated salts,
which would prohibit its use in agriculture applications.
Elimination of organic waste from marine RASs using
resident microbial communities is currently under study,
with a focus on anaerobic digestion reactors that promote
activity of methanogenic Archaea [5]. Although high salt
concentrations associated with marine RAS sludge may
inhibit methane-producing Archaea, methanogenic
activity has been detected for several marine RASs at
levels lower than those observed for domestic and industrial
sludge digestion [40]. The archaeal community in
brackish aquaculture sludge digesters consist of members
of both Euryarchaeota and Crenarchaeota, with evidence for
methanogenic representatives of the Methanosarcinaceae
obtained from 16S rRNA gene sequences (Mirzoyan and
Gross, manuscript in preparation).